Fuel injection systems allow control and optimization of the quantity of fuel injected into the combustion chambers of an engine, the timing of fuel delivery with respect to the crankshaft and piston position, and the presentation of fuel to the combustion chamber, for example by atomising and dispersing the fuel in a pre-determined pattern. Modern fuel injection systems use electronic controls to achieve a high level of precision in the quantity and timing of the fuel delivery. This high precision is required to meet emissions and performance expectations of the marketplace.
Common rail fuel injection systems are well known, particularly in the field of compression ignition engines such as diesel engines. A typical common rail fuel injection system for an automobile is shown schematically in FIG. 1 of the accompanying drawings. Fuel is stored in a fuel tank 20, and is drawn by way of a lift pump 22 and a filter 24 to an engine-driven high-pressure pump 26. The high-pressure pump 26 supplies fuel at elevated pressure to an accumulator or rail 28. Fuel injectors 30 are connected to the rail by respective jumper pipes 32. Each fuel injector 30 is arranged to supply fuel to one respective cylinder of the engine by injecting the fuel into a combustion chamber of the cylinder under the control of an electronic control unit (ECU) 34.
Many types of fuel injector are known. In a typical arrangement, a fuel injector includes a control valve comprising a valve needle moveable between a first position and a second position upon actuation of an actuator, for example a solenoid or a piezoelectric actuator. The valve needle is accommodated within a body of the fuel injector. The body defines a nozzle provided with at least one orifice downstream of a seating surface for the valve needle. The seating surface, in turn, lies downstream of a reservoir of fuel at high pressure. In the first position, the valve needle seals against the seating surface, so as to prevent flow of fuel past the seating surface. In the second position, the valve needle is held away from the seating surface, so that fuel can flow from the reservoir, through the or each orifice and into the combustion chamber, thus effecting an injection of fuel.
The quantity of fuel delivered to the combustion chambers affects the torque output of the engine. Consequently, fuel delivery must be carefully controlled to provide the desired torque output at any given time under the conditions then prevailing.
The quantity of fuel delivered over the course of each injection event is a function of the nozzle orifice flow area, the fuel pressure and the injection duration. The injection duration is the time over which the needle is lifted from the seating surface, so that high-pressure fuel can flow into the combustion chamber through the orifice.
In a given fuel injector, the nozzle orifice flow area is fixed. Fuel delivery is therefore controlled using the so-called ‘pressure-time’ principle. To achieve delivery of a desired quantity of fuel, the injection duration is set electronically to a value which has been pre-calculated so that, assuming a certain fuel pressure, the required quantity of fuel will pass into the combustion chamber over the time that the fuel can flow through the nozzle, i.e. the injection duration. Consequently, any unintended variation in the fuel pressure may result in an incorrect quantity of fuel being delivered to the combustion chamber, with the result that the engine produces an output torque which is more or less than required. In these circumstances, the driveability, performance and emissions of the vehicle may be compromised.
Referring again to FIG. 1, control of the injection timing and duration is achieved by the ECU 34. The ECU 34 accepts input signals from a variety of sensors, which may include a crankshaft speed sensor 36a, a crankshaft phase sensor 36b, a throttle pedal demand sensor 36c, an air intake temperature sensor 36d, a coolant temperature sensor 36e, an air intake mass flow sensor 36f, and, in turbocharged engines, an intake boost pressure sensor 36g. In addition, common rail fuel injection systems include a fuel rail pressure sensor 38, which may be combined with a fuel temperature sensor. The ECU 34 controls, by way of output signals, various actuators which actuate a metering flow valve 40 at the inlet of the high-pressure pump 26, a rail pressure control valve 42, and control valves of the individual injectors 30.
The rail pressure sensor 38 is typically a piezo-resistive device with integrated electronics. It is installed intrusively in the rail 28, so that a portion of the sensor body, typically a diaphragm, is directly exposed to the high-pressure fuel in the rail 28. Generally, the rail pressure sensor 38 is screwed into a threaded port 44 in the rail 28, and a soft iron washer may be used to effect a seal between the sensor 38 and the rail 28. As rail pressure sensors 38 must operate reliably and without leakage in a very high-pressure environment, such sensors 38 are relatively expensive and delicate.
The nominal fuel pressure in the rail 28, and hence in each fuel injector 30, is determined by the ECU 34 using the input signals from the sensors 36a-36g, 38 to determine the engine operating conditions and the torque requirement. For example, at low engine speeds and low loads, the nominal rail pressure may be 300 bar; while at high engine speeds and high loads the nominal rail pressure may be 2000 bar. Typically, a range of optimum nominal rail pressures is recorded for a corresponding range of conditions in a calibration procedure during engine set-up and testing. The optimised values are determined so as to minimise emissions, optimise performance, or minimise fuel consumption as required. These optimised nominal pressures are stored in a map in a memory of the ECU 34 so that the optimised value for a given engine condition can be retrieved.
Under a given set of engine conditions, therefore, the nominal mean rail fuel pressure has a fixed value. The ECU 34 determines the actual, instantaneous rail fuel pressure from the rail pressure sensor 38, and operates the inlet metering flow valve 40 of the high-pressure fuel pump 26 or the rail pressure control valve 42 as appropriate to achieve and maintain the desired mean rail fuel pressure. In this way, a feedback control system is provided. Sophisticated control algorithms are provided to optimise this feedback control system. It is important that the rail pressure sensor is as accurate as possible because unexpected variations in rail fuel pressure will cause unexpected variations in torque output.
The response time of the feedback control system is limited by the performance of the rail pressure sensor 38, the ECU 34, the high-pressure pump 26 and the inlet metering flow valve 40 or the rail pressure control valve 42. For example, if the rail pressure drops, the rail pressure sensor 38 must respond to the pressure drop by sending an appropriate signal to the ECU 34, the ECU 34 must then evaluate the signal and respond by actuating the inlet metering flow valve 40, and within the constraints of its flow capacity, the high-pressure pump 26 must increase the rail pressure to the required value.
An injection event places an instantaneous flow demand on the fuel volume stored in the rail 28. The instantaneous flow demand is such that the control system cannot respond rapidly enough and, as a consequence, the fuel pressure in the rail 28 drops. The fuel pressure in the rail 28 is therefore perturbed, and a short time elapses before the pressure recovers to the desired level, although this recovery is hopefully complete before the next injection event. The drop in pressure means that, over the duration of a normal injection event, the mean pressure in the rail 28 may be slightly below the target pressure, but this effect can be accounted for during calibration so that the anticipated torque is still achieved.
Recent developments in fuel injection technology, and of common rail systems in particular, have introduced the capability of delivering fuel in multiple injection events per combustion cycle. In other words, instead of a single injection event occurring during each cycle of the cylinder, the fuel is delivered in a sequence, or train, of two or more precisely timed injection events, each of which injects a carefully controlled quantity of fuel. For example, an injection sequence may comprise a pilot injection or pre-injection, which pre-heats the gases in the combustion chamber ahead of a main injection in which the majority of the fuel is injected. A post injection, after the main injection, may also be provided to encourage complete combustion of unburnt fuel, thus reducing harmful exhaust emissions and improving fuel efficiency.
Modern engines, therefore, may utilise multiple injection events per cycle to optimise performance and fuel efficiency and to reduce harmful exhaust emissions. Over a range of engine load and speed conditions, the optimum injection sequence may change. For example, some conditions may require a pilot injection closely followed by a main injection, some conditions may require a split main injection, other conditions may require pilot, main and post injections, while still other conditions may require multiple pilot or multiple post injections.
When sequences of multiple injection events are required, the possibility arises that a perturbation to the rail pressure, and hence to the fuel pressure in the injectors 30, caused by a prior injection event may still be present when a subsequent injection event begins. In other words, the pressure wave within the fuel system that results from the prior injection event may not have died away when the subsequent injection event occurs. Consequently, the fuel pressure in the injector 30 at the time of the subsequent injection event is not at the expected level, corresponding to the target rail pressure. Instead, the pressure in the injector 30 is lower or higher than the expected pressure, depending on the phase relationship of the pressure wave to the subsequent injection event. In either case, the result is that an incorrect, unpredicted and unpredictable quantity of fuel is delivered, with similarly unpredictable consequences for torque output and emissions.
Significant errors in the fuel quantity delivered can arise because of this phenomenon, and these errors can result in unacceptable emissions, increased noise, impaired driveability, poor performance and so on.
One known approach to reduce or mitigate the undesired effects of these residual pressure waves involves providing tuning orifices at particular locations in the fuel system to damp the pressure waves resulting from injection events, thus impeding propagation of the waves. However, this approach is inflexible because the tuning orifices are effective only over a relatively limited range of engine conditions and injection sequences. In particular, this approach is of limited value where more than one injection strategy is employed in a given engine.
The effects of pressure waves in multiple-injection sequences could, in theory, be compensated for by mapping the entire speed and load regime of the engine with fine granularity and calibrating the injection durations in the sequence to compensate for residual pressure waves. However, this approach is impractical because it would require an extremely laborious calibration procedure, as well as the storage and rapid retrieval of a huge amount of data by the ECU. Furthermore, the calibrated injection durations would be sensitive to minor changes in pipe lengths and build tolerances.
It is against this background that the present invention has been devised.